feat: system messages, group DM welcome header, and dm_owner_updated event

- Render system messages (member_added, member_removed, owner_changed) inline
  in MessageList with icon + human-readable text; system messages never group
  with adjacent user messages
- Rewrite WelcomeHeader to branch on ownerId: group DMs show overlapping avatars,
  group name, creator attribution, federated privacy note, and a Leave Group button
- Add dm_owner_updated ServerEvent; broadcast from dm.ts leave handler and
  federation processOwnershipTransferEvent so all clients update ownerId in real-time
- Add updateDmOwner action to spaceStore and handle dm_owner_updated in useWebSocket
- Add leaveGroup alias to API client dm namespace

packages/web/src/api/client.ts

@@ -145,6 +145,7 @@ export class BackspaceApiClient {
     deleteMessage: (id: string) => Promise<{ success: boolean }>;
     addMember: (dmChannelId: string, data: { userId: string }) => Promise<DmChannel>;
     leave: (dmChannelId: string) => Promise<{ success: boolean }>;
+    leaveGroup: (dmChannelId: string) => Promise<{ success: boolean }>;
   };
 
   readonly social: {
@@ -505,6 +506,8 @@ export class BackspaceApiClient {
         request<DmChannel>('POST', `/dm/${dmChannelId}/members`, data),
       leave: (dmChannelId: string) =>
         request<{ success: boolean }>('DELETE', `/dm/${dmChannelId}/members`),
+      leaveGroup: (dmChannelId: string) =>
+        request<{ success: boolean }>('DELETE', `/dm/${dmChannelId}/members`),
     };
 
     this.social = {